Advice is a suggestion about what to do. If you just can't decide between the pecan pie and the chocolate torte, ask the waiter for his advice.
Advice comes from old French words for seeing, or view. It first meant opinion, but came to mean an opinion about what to do. If you become President, you'll gather a cabinet to give you their advice. If your doctor tells you to eat better, exercise more, and take up swimming, that's her medical advice. Someone gives you their advice, never "an advice." You can, however, ask for a "piece of advice."